2. The Body’s Defenses The Body’s Defenses

4. Barriers To Infection: Our First Line of Defense- Stopping the invasion  The body has main barriers to infection that may stop pathogens from ever getting into the body, causing infection. These are the Skin, Breathing Passages, the Mouth/Stomach.  1.) Skin: Acts like a barrier several ways. First, chemicals in the oils and sweat on your skin kill many pathogens when they land on it. Second, as the surface layer of dead skin cells flake off they take many pathogens with them. Finally, the epidermis’s tough layers of dead cells acts like a shield keeping many pathogens out.

5. Barriers (cont.)  2.) Breathing Passages: These act as barriers in two ways.  First, mucus traps pathogens and cilia (little hair-like structures) sweep away most pathogens trying to enter.  Second, by coughing or sneezing we expel many pathogens.

7. Barriers (cont.)  3a.) The Mouth: saliva contains chemicals that kill some pathogens.  3b.) The Stomach: Many of the pathogens that survive your saliva wind up in your stomach when you swallow and are destroyed by your stomach acids.

8. The Inflammatory Response:Our Second Line of Defense  Ok, germs got past our first line of defense and inside us. Now What?  When body cells are damaged by invading germs they release chemicals. These chemicals trigger our second line of defense- The Inflammatory Response to kick in.  First, blood vessels enlarge releasing special white blood cells into affected tissues that get involved during the inflammatory response called phagocytes which engulf and destroy pathogens.  Second, these chemicals also may cause a fever, which often stops the growth and reproduction of pathogens, allowing phagocytes to clear them from the body.

10. The Immune Response:Our Third Line of Defense The Immune Response:Our Third Line of Defense  If an infection is severe enough to cause a fever, our third line of defense is triggered- The Immune Response.  The special white blood cells of the Immune system can tell the difference between different types of pathogens and develop a defense specifically targeting that pathogen.  The Immune system’s special white blood cells are called Lymphocytes.

11. The Immune Response (cont.) TTTThere are two different types of Lymphocytes; T lymphocytes (or T cells) and B lymphocytes (B cells). They each have different jobs but work together to destroy pathogens. WWWWe all have tens of millions of T cells circulating in our bloodstream. When a T cell encounters a pathogen it identifies it based on unique marker molecules that each type of pathogen has on its surface. These marker molecules are called Antigens.

12.  All cells have antigens. Each person has identical antigens which tell a T cell whether a cell it encounters is yours or an invader.  When T cells identify an invader they activate B cells to produce Antibodies for that pathogen. . Antibodies are chemicals that lock onto a pathogen’s antigens and mark them (like signs) for destruction by phagocytes.

15. AIDS: A Disease of the Immune System  Is the only disease where viruses attack the immune system directly.  Is caused by the HIV (Human Immunodeficiency Virus).  It invades and destroys T cells. Without their protection, you have no protection against other invading pathogens, resulting in illness and later death.

16. AIDS: (continued)  It is spread only through the transmission of body fluids (blood or human reproductive fluids) from an infected person to an uninfected person.  This can occur as a result of sex, infected mothers passing the virus to her infant in the womb or when nursing, or through the sharing of needles by intravenous drug users.